1. Field of the Invention
The present invention relates to the production of the cyclic trimmer of formaldehyde, and it relates in particular to the separation of said trimer from the aqueous solutions containing it together with formaldehyde.
2. Description of the Prior Art
The cyclic trimer of formaldehyde is commonly known as trioxane, and this name will be used in the following description. As is known, trioxane is formed in concentrated aqueous solutions of formaldehyde when acidic substances are added to such solutions (Journal of the Chemical Society - London 121, 1922). In this way an equilibrium is established between the formaldehyde and its timer, the said equilibrium depending on the initial concentration of formaldehyde in the solution. The concentrated aqueous solutions of formaldehyde that are suitable for the production of trioxane can be obtained by evaporation of less concentrated solutions, for example commercial formalins, or by dissolutions of low polymers of formaldehyde, such as paraformaldehyde, in water.
The methods used for the separation of the trioxane from the reaction medium are generally based on the distillation of an azeotrope with water. In the absence of formaldehyde this azeotrope consists of 70% by weight of trioxane and 30% by weight of water, with a boiling point of 91.3.degree. C at atmospheric pressure. The trioxane is recovered from the distillate by extraction with a solvent that is immiscible with water, followed by fractional distillation of the organic phase obtained in this way.
A disadvantage of this process is that more or less large quantities of formaldehyde, depending on the composition of the starting mixture, distil during the distillation of the trioxane-water azeotrope (see U.S. Pat. No. 2,347,447), and the composition of the distillate also varies according to the distillation rate.
Thus industrially acceptable distillation rates are obtained if temperatures of from 93.degree. to about 97.degree. C are maintained at the top of the column. Under these conditions the quantity of trioxane in the distillate lies within a range of values from about 20 to about 40% by weight, while the quantity of formaldehyde varies from about 10 to about 40% by weight. When temperatures of about 91.degree. to about 93.degree. C are maintained at the top of the distillation column, a distillate that is richer in trioxane is obtained, but the distillation rate decreases so that it becomes industrially unacceptable.
A decrease in the content of formaldehyde and an increase in the content of trioxane at the top of the distillation column are therefore obtained if high reflux ratios are maintained, the said ratios generally being greater than 3.
However, this procedure has various disadvantages, such as the deterioration of the energy balance in relation to the trioxane produced and the long residence time at the distillation temperatures, with the result that the formation of by-products derived from formaldehyde through secondary reactions, such as the Cannizzaro reaction (formation of methanol and formic acid) and the Tishchenko reaction (formation of methyl formate), is favored.
A further disadvantage of the said distillation procedure is that the formaldehyde that rises to the top of the column is readily deposited in the form of a polymer on the cooler parts of the apparatus. This phenomenon, which is observed especially in processes in which high distillation rates are used, causes incrustation and blockage of pipes and similar parts. It therefore follows that the operation of the column has to be interrupted to remove the deposited polymer, e.g., by treatment with steam. The deposition of the polymer is related not only to the total concentration of formaldehyde but also to the concentration of trioxane in the fraction that separates at the top of the distillation column. In addition to the undesirable interruptions, therefore, the processes of prior art have the further disadvantage inherent in the loss of rather considerable quantities of formaldehyde.
This last disadvantage can be partly avoided, according to a known technique (see Italian Pat. No. 671,941) by introduction of lower aliphatic alcohols such as methanol and ethanol, or of their hemiformals, into the boiling system. Owing to the range of conditions used, however, reaction by-products such as methylals, which, when methanol is used, have the following general formula: EQU CH.sub.3 --O(CH.sub.2 O).sub.n CH.sub.3
where n generally has values of from 1 to 3, are formed. This fact, besides leading to losses of formaldehyde, does not allow the production of trioxane with a high purity. This is because the separation of the methylals is very difficult owing to a certain chemical affinity for trioxane and also owing to the closeness of certain physical constants such as the boiling point and those connected with the extractability with solvent.
On the other hand, the trioxane is mainly used for the production of acetal homopolymers and copolymers, for which a monomer of very high purity is required both in the case of cationic polymerization and in the case of free-radical polymerization.
Another limiting factor in the known processes is the fact that the distillation rate is determined by the kinetics of formation of trioxane in the reaction mixture contained in the boiler of the column, the said boiler also acting as the trimerization reactor, in which, for the system in question, it is convenient to work with trioxane concentrations very close to the equilibrium concentration. It is therefore necessary to maintain sufficiently long residence times so that as the distillation proceeds, a trioxane concentration such as to ensure a constant feed to the distillation column is formed in the reaction mixture.